Abstract
Comment sections on social media extend social influence beyond offline networks, allowing a small, vocal minority of users to reach much larger audiences. We provide causal evidence that the views expressed in comments below social media posts shape both on-platform engagement and off-platform attitudes and behavior, and that these effects move in opposite directions. In collaboration with a leading racial justice organization, we conduct a large-scale field experi ment on Facebook reaching a million U.S. users randomly assigned to one of four conditions: (i) no visible comments (control), (ii) opposing, (iii) supportive, and (iv) mixed comments dis playing both stances. Opposing comments increase reactions, comments, and link clicks by roughly 15-45% relative to the control, whereas supportive comments have little effect. A com plementary survey experiment with 5,000 participants shows that the same opposing comments shift attitudes in a less progressive direction and reduce donations to the organization by 7.3%. These results reveal a fundamental trade-off: the same comments that increase on-platform engagement undermine off-platform influence.